Recent advances have been made in the field of producing three-dimensional objects, such as prototype parts and finished parts in small quantities, directly from computer-aided-design (CAD) data bases. Various technologies are known to produce such parts, particularly through the use of additive processes, as opposed to subtractive processes such as conventional machining. An important additive process for the production of such objects is selective laser sintering, recently commercialized by DTM Corporation. According to the selective laser sintering process, a powder is scanned in layerwise fashion by a directed energy beam, such as a laser, to fuse the powder at selected locations corresponding to cross-sections of the object. Fused locations within each layer adhere to fused portions of previously fused layers, so that a series of layers processed in this manner results in a finished part. Computer control of the scanning of the energy beam thus enables direct transfer of a design in a computer-aided-design (CAD) data base into a physical object.
The selective laser sintering technology is described in further detail in U.S. Pat. No. 4,247,508 issued Jan. 27, 1981, now assigned to DTM Corporation and incorporated herein by reference, and in U.S. Pat. No. 4,863,538 issued Sep. 9, 1989, U.S. Pat. No. 5,017,753 issued May 21, 1991, U.S. Pat. No. 4,938,816 issued Jul. 3, 1990, and U.S. Pat. No. 4,944,817 issued Jul. 31, 1990, all assigned to Board of Regents, The University of Texas System and also incorporated herein by this reference. As described in the above-noted patents, and also in U.S. Pat. No. 5,156,697 issued Oct. 20, 1992, U.S. Pat. No. 5,147,587 issued Sep. 15, 1992, and U.S. Pat. No. 5,182,170 issued Jan. 26, 1993, all also assigned to Board of Regents, The University of Texas System and incorporated herein by this reference, various materials and combinations of materials can be processed according to this method, such materials including plastics, waxes, metals, ceramics, and the like. In addition, as described in these patents and applications, the parts produced by selective laser sintering may have shapes and features which are sufficiently complex as to not be capable of fabrication by conventional subtractive processes such as machining. This complexity is enabled by the natural support of overhanging fused portions of the object that is provided by unfused powder remaining in prior layers.
Further refinements in the selective laser sintering process, and advanced systems and machines for performing selective laser sintering, are described in U.S. Pat. No. 5,155,321 issued Oct. 13, 1992, commonly assigned herewith, U.S. Pat. No. 5,155,324 issued Oct. 13, 1992, and International Publication WO 92/08592, all of which are incorporated herein by reference. Copending application Ser. No. 789,358, filed Nov. 8, 1991, commonly assigned herewith and incorporated herein by this reference, further describes an advanced apparatus for selective laser sintering in which powder is dispensed from either side of the target surface.
The selective laser sintering process is primarily a thermal process, as the object is formed by the sintering or other fusing of powder at selected locations of a layer that receive directed energy from the laser sufficient to reach the fusing or sintering temperature. Those portions of each powder layer that do not receive the laser energy are to remain unfused, and thus must remain below the fusing or sintering temperature. In addition, the temperature of the powder receiving the laser energy will generally be higher than the temperature of underlying prior layers (fused or unfused). As such, significant thermal gradients are present at the target surface of the powder in the selective laser sintering process.
It has been observed that these thermal gradients can result in distortion of the object being produced, thus requiring thermal control of the selective laser sintering process in order for the objects produced to precisely meet the design. One cause of such distortion is warpage and shrinkage of the object due to thermal shrinkage of the sintered layers it cools from the sintering temperature to its post-sintering temperature; in addition, shrinkage can occur due to the reduction in volume of the fused powder as it passes through the phase change from liquid to solid. In either case, the reduction in volume of the sintered powder will cause the top of the object to contract. Since underlying layers have already contracted, being immersed in the fairly good thermal insulator of unfused powder, tensile stress is induced at the surface, and curling of the object can result.
Another source of distortion in the production of objects by selective laser sintering is undesired growth of the part being produced beyond the volume defined by the laser beam. As is well known, the spot size of a laser beam can be made quite small so that the resolution of features in the object can be quite sharp. However, conduction of heat from the fused locations can cause powder outside of the scan to sinter to the directly sintered portion, causes the fused cross-section to "grow" beyond the area of the laser scan, and thus beyond the design dimensions. Growth can also occur from layer to layer if sufficient heat from sintering remains in the fused portion that newly dispensed powder (in the next layer) sinters to the prior layer as it is dispensed.
As described in the above-referenced U.S. Pat. No. 5,017,753 and U.S. Pat. No. 5,155,321, control of the temperature of the sintered and unsintered powder at the target surface in the selective laser sintering process is important in minimizing such distortion in the object being produced. It has been newly discovered, however, that the manner in which the laser scans the selected portion of each layer to be fused in fabricating the part is a large factor in the consistency of the formation of the cross-section of the object being formed.
Another laser-based process for forming of three-dimensional objects is commonly referred to as stereolithography. According to the stereolithography technique, as described in U.S. Pat. Nos. 4,575,330 and 4,929,402, a directed light beam such as a laser is used to cure selected portions of the surface of a vat of photopolymer. Considering the heat produced by the irradiation of any material with a laser, as well as the heat released in polymerization, it is contemplated that thermal effects are also present at the surface of objects formed according to the stereolithography process, and that result in distortion. It is further contemplated that such effects are in large part due to the energy imparted by the laser and released from the polymerization, the amount of such energy differing among the various locations within the layer.
By way of further background, in conventional selective laser sintering systems, the laser scans the target surface in a raster scan mode; as described in the above-referenced U.S. Pat. No. 5,155,324, such raster scanning may be done in combination with outlining of the cross-section of the object to be formed by the laser in a vector mode. In performing the raster scan, these conventional systems scan across substantially the entire target surface in one dimension, turning the laser on and off at the boundaries of the cross-section, regardless of the size, shape or location of the article being formed; in addition, the scans are incremented in such a manner as to overlap the prior scan. Accordingly, the time between adjacent and overlapping scans of the cross-section of the object being formed may widely vary according to the position of the object in the part bed, the size of the cross-section, and the direction of scan. It is believed that such variations in this time is the cause of inconsistent object formation, including variations in texture and distortion effects.
It is therefore an object of the present invention to control the thermal effects of the laser scan within the fused locations in each layer, so that the thermal effects of the laser heat are made consistent.
It is a further object of the present invention to so control these thermal effects in a manner consistent with the formation of multiple objects in a single build cycle.
It is a further object of the present invention to so control these thermal effects while minimizing the scan time of the laser for each layer of powder.
It is a further object of the present invention to so control these thermal effects for both powder-based and liquid-based processes.
Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification together with the drawings.